Tell them no, but use diplomacy. For those who ask, tell them, the pond is only for your immediate family. Your family has wanted a private fishing pond for themselves, that's one of the main reasons you moved there. State that it's easier on yourself to post it as private, with no trespassing, that way everyone is treated equal. Make sure to add an alternative like "there's always lake xxx, it's a few minutes away and has fish much larger than the ones in your pond."
If you're still talking after that, mix in a story about the excessive pond moss and the off taste of your fish. (I'm not encouraging you to lie...but fresh fish don't taste like store bought !)
